Bacterium Shewanella Oneidensis MR-1 Research Articles

Evolution of paralogous multicomponent systems for site-specific O-sialylation of flagellin in Gram-negative and Gram-positive bacteria

Many bacteria glycosylate flagellin on serine or threonine residues using pseudaminic acid (Pse) or other sialic acid-like donor sugars. Successful reconstitution of Pse-dependent sialylation by the conserved Maf-type flagellin glycosyltransferase (fGT) may require (a)missing component(s). Here, we characterize both Maf paralogs in the Gram-negative bacterium Shewanella oneidensis MR-1 and reconstitute Pse-dependent glycosylation in heterologous hosts. Remarkably, we uncovered distinct acceptor determinants and target specificities for each Maf. Whereas Maf-1 uses its C-terminal tetratricopeptide repeat (TPR) domain to confer flagellin acceptor and O-glycosylation specificity, Maf-2 requires the newly identified conserved specificity factor, glycosylation factor for Maf (GlfM), to form a ternary complex with flagellin. GlfM orthologs are co-encoded with Maf-2 in Gram-negative and Gram-positive bacteria and require an invariant aspartate in their four-helix bundle to function with Maf-2. Thus, convergent fGT evolution underlies distinct flagellin-binding modes in tripartite versus bipartite systems and, consequently, distinct O-glycosylation preferences of acceptor serine residues with Pse.

Abstract 1201 "Isolation and Characterization of Bacteriophages Infecting Metal Iron-reducing Bacterium Shewanella oneidensis MR-1"

Abstract 1201 "Isolation and Characterization of Bacteriophages Infecting Metal Iron-reducing Bacterium Shewanella oneidensis MR-1"

Clay-bacteria interaction: Effect of bacterial cell density on sedimentation behavior and fabric map of kaolinite clay

Clay-bacteria interactions are ubiquitous and the presence of bacteria has a pronounced effect on the mechanical, chemical, and electrical behaviors of clays. However, the role of microscale clay-bacteria interactions to the macro-scale engineering behaviors and properties of clays remains poorly understood. This study presents changes in the microstructure of kaolinite in association with bacteria and the resulting sedimentation behavior of bacterium-associated kaolin suspensions. Sedimentation tests were performed using suspensions composed of kaolin and model bacteria Shewanella oneidensis MR-1 while varying the fluid pH and cell density. The results reveal that kaolinite presents face-to-face aggregated microstructures with the presence of bacteria, attributable to sharing and/or exchange of cations between kaolinite and bacterial cells. Meanwhile, at a pH value lower than the isoelectric point of the kaolinite edge surfaces, the clay-bacteria association appears less pronounced comparing to the higher pH, owing to the competition between negative (kaolinite faces and bacteria surfaces) over positive (kaolinite edges) charges. The fabric change caused by kaolinite-bacteria association accelerates kaolin settling and reduces the final void ratios. A fabric map for bacterium-asssociated kaolinite is suggested based on the sedimentation behaviors and scanning electron microscopy observations. This study advances the understanding of particle-level clay-bacteria interactions and the sedimentation behavior of clays in suspensions with vigorous microbial activities.

Nanoscale Carbonate Ion-Selective Amperometric/Voltammetric Probes Based on Ion-Ionophore Recognition at the Organic/Water Interface: Hidden Pieces of the Puzzle in the Nanoscale Phase.

Here, we report on the successful demonstration and application of carbonate (CO32-) ion-selective amperometric/voltammetric nanoprobes based on facilitated ion transfer (IT) at the nanoscale interface between two immiscible electrolyte solutions. This electrochemical study reveals critical factors to govern CO32--selective nanoprobes using broadly available Simon-type ionophores forming a covalent bond with CO32-, i.e., slow dissolution of lipophilic ionophores in the organic phase, activation of hydrated ionophores, peculiar solubility of a hydrated ion-ionophore complex near the interface, and cleanness at the nanoscale interface. These factors are experimentally confirmed by nanopipet voltammetry, where a facilitated CO32- IT is studied with a nanopipet filled with an organic phase containing the trifluoroacetophenone derivative CO32-ionophore (CO32-ionophore VII) by voltammetrically and amperometrically sensing CO32- in water. Theoretical assessments of reproducible voltammetric data confirm that the dynamics of CO32- ionophore VII-facilitated ITs (FITs) follows the one-step electrochemical (E) mechanism controlled by both water-finger formation/dissociation and ion-ionophore complexation/dissociation during interfacial ITs. The yielded rate constant, k0 = 0.048 cm/s, is very similar to the reported values of other FIT reactions using ionophores forming non-covalent bonds with ions, implying that a weak binding between CO32- ion-ionophore enables us to observe FITs by fast nanopipet voltammetry regardless of the nature of bondings between the ion and ionophore. The analytical utility of CO32--selective amperometric nanoprobes is further demonstrated by measuring the CO32- concentration produced by metal-reducing bacteria Shewanella oneidensis MR-1 as a result of organic fuel oxidation in bacterial growth media in the presence of various interferents such as H2PO4-, Cl-, and SO42-.

Soluble Iron Enhances Extracellular Electron Uptake by Shewanella oneidensis MR-1.

Extracellular electron transfer (EET) is a process that microorganisms use to reduce or oxidize external insoluble electron acceptors or donors. Much of our mechanistic understanding of this process is derived from studies of transmembrane cytochrome complexes and extracellular redox shuttles that mediate outward EET to anodes and external electron acceptors. In contrast, there are knowledge gaps concerning the reverse process of inward EET from external electron donors to cells. Here, we describe a role for soluble iron (exogenous FeCl2) in enhancing EET from cathodes to the model EET bacterium Shewanella oneidensis MR-1, with fumarate serving as the intracellular electron acceptor. This iron concentration-dependent electron uptake was eradicated upon addition of an iron chelator and occurred only in the presence of fumarate reductase, confirming an electron pathway from cathodes to this periplasmic enzyme. Moreover, S. oneidensis mutants lacking specific outer membrane and periplasmic cytochromes exhibited significantly decreased current levels relative to wild-type. These results indicate that soluble iron can function as an electron carrier to the EET machinery of S. oneidensis.

Biogenic synthesis of palladium nanoparticles: New production methods and applications

Abstract The palladium (Pd)-catalysed reaction has attracted much attention, making Pd the most valuable of the four major precious metals. Several different forms of Pd can be used as a catalyst; nanoparticles (NPs) have the advantage of a high surface area:volume ratio. Since the chemical production of Pd NPs is not environmentally friendly, biological synthesis interest has grown. However, the production mechanism remained unknown in several cases and was recently described for the electroactive bacterium Shewanella oneidensis MR-1. The application of these green synthesised NPs was established in different fields. This review discusses the production pathway and the novel biological-inspired methods to produce tailored biogenic palladium nanoparticles (bio-Pd NPs), with their broad application fields as biogenic nanocatalysts. Two significant applications – reductive bioremediation of persistent organic contaminants and energy-producing microbial fuel cells – are discussed in detail. The current challenges in optimising bio-Pd NPs production and the potential research directions for the complete utilisation of its novel catalytic properties are highlighted.

Remediation of arsenic-containing ferrihydrite in soil using iron- and sulfate-reducing bacteria: Implications for microbially-assisted clean technology

The iron-reducing bacterium (IRB) Shewanella oneidensis MR-1 and a sulfate-reducing bacterium (SRB) Desulfovibrio vlugaris miyazaki, were used in varying sequences for batch and column arsenic extraction experiments to assess their use in alleviating arsenic pollution from arsenic-containing ferrihydrite in soil. Both batch (four conditions) and column experiments show variations in biological sequence affect the arsenic release and species. The results of arsenic release from batch conditions are: IRB-SRB (2.28 mg/g) > SRB-IRB (1.72 mg/g) > SRB+IRB (0.55 mg/g) > Control group (pure water, 0.04 mg/g). Further cycle operations from batch experiments showed that IRB-SRB operation always obtained the best performance of releasing arsenic. The column experiments achieved greater arsenic release: SRB+IRB (10.87 mg/g) > IRB-SRB (7.54 mg/g) > SRB-IRB (6.48 mg/g) relative to arsenic in the pristine samples (36.6 mg/g). Flow methods could obtain more releasing performance than batch experiment. The hydraulic conductivity in the column experiments was best in the SRB-IRB operation (average water difference: 0.39 psi for SRB-IRB, 0.86 psi for IRB-SRB and 1.07 psi for SRB+IRB). After the biological operation, chemical treatment (0.5 M, (NH4)2HPO4) only desorbed 11.14–39.81% of the total arsenic quantity of each operation. Reductive dissolution is the main driving reason for arsenic release, though the formation of FeS was found to hinder arsenic release. Analysis of the solids showed that the SRB+IRB operation results in 58.07% arsenic released. These results suggest that microbially-assisted reduction using iron- and sulfate-reducing bacteria may be an effective technology for remediating arsenic-bearing soil e.g. paddy soil using systems such as an in-situ injection and pump.

Riboflavin-rich agar enhances the rate of extracellular electron transfer from electrogenic bacteria inside a thin-layer system

Numerous bacteria owe extracellular electron transport (EET) ability, and the rate enhancement of EET is critical for the emerging sensor technology to detect metabolically active pathogens. Here, the considerable enhancement of microbial current signal was firstly demonstrated in a thin layer electrolyte sandwiched between an agar substrate (AS) containing high concentration riboflavin (RF) and a screen-printed electrode. Covering cells with this AS showed a sharply current increase from 0.033 µA to 1.59 μA (47.7-folds) in EET-capable bacteria Shewanella oneidensis MR-1. Differential pulse voltammograms using gene-deletion mutant strains of S. oneidensis MR-1 revealed thin electrolyte between RF-loaded AS and electrode enhanced the rate of electron transfer via complexes between riboflavin and outer membrane c-type cytochrome. A similar effect in Streptococcus mutans UA159, a biofilm-forming pathogen, was also explored. Moreover, capturing and quantifying both metabolically active microbes from the dry solid surface are demonstrated with RF-loaded AS successfully. The considerable enhancement of the EET in the thin layer electrolyte provides a new direction for designing whole-cell biosensors and understanding a microbe/electrode interaction in a micro-sized space.

Enabling Electron Injection for Microbial Electrosynthesis with n-Type Conjugated Polyelectrolytes.

Microbial electrosynthesis-using renewable electricity to stimulate microbial metabolism-holds the promise of sustainable chemical production. A key limitation hindering performance is slow electron-transfer rates at biotic-abiotic interfaces. Here a new n-type conjugated polyelectrolyte is rationally designed and synthesized and its use is demonstrated as a soft conductive material to encapsulate electroactive bacteria Shewanella oneidensis MR-1. The self-assembled 3D living biocomposite amplifies current uptake from the electrode ≈674-fold over controls with the same initial number of cells, thereby enabling continuous synthesis of succinate from fumarate. Such functionality is a result of the increased number of bacterial cells having intimate electronic communication with the electrode and a higher current uptake per cell. This is underpinned by the molecular design of the polymer to have an n-dopable conjugated backbone for facile reduction by the electrode and zwitterionic side chains for compatibility with aqueous media. Moreover, direct arylation polycondensation is employed instead of the traditional Stille polymerization to avoid non-biocompatible tin by-products. By demonstrating synergy between living cells with n-type organic semiconductor materials, these results provide new strategies for improving the performance of bioelectrosynthesis technologies.

Insight into the magnetic properties of Pb-dopped iron oxide nanoparticles during Fe(III) bio-reduction by Shewanella oneidensis MR-1

Anthropogenic activities have led to a significant accumulation of Pb in the environment, posing a threat to ecosystems. Iron oxides that display magnetic properties are ubiquitous in the environment and Pb partitioning onto these minerals is considered one of the most critical geochemical processes controlling its environmental fate. In anoxic environments, iron oxides undergo redox cycling due to biotic and abiotic routes, resulting in their transformation/dissolution and evolution of their magnetic characteristics. However, there is still a lack of knowledge on the impact of Pb on the dynamic of iron oxides bioreduction. Furthermore, there is little information available regarding the nature of biogenic minerals and their magnetic signatures.Here we incubated Pb-bearing ferrihydrites (Fh_Pb) with various Pb/(Fe + Pb) molar ratios (i.e., 0, 2 and 5%) with the iron-reducing bacterium Shewanella oneidensis MR-1, for 21 days.XRD analyses of the initial Fh_Pb displayed characteristic features of Fh and a decrease of magnetization in the presence of Pb. During the bio-reduction process, Pb doping led to a drop in the rate and extent of reduction. At the end of the incubation period, the analysis of the aqueous solutions revealed a minor proportion of Pb in solution, indicating that a significant proportion of the Pb is sorbed onto the biogenic minerals. Magnetite (95%) and siderite (5%) formed during the bioreduction of Fh, while magnetite (~80%) and goethite (~20%) precipitated in the presence of Pb as revealed by transmission Mössbauer spectroscopy. Furthermore, the size of the magnetite particles decreased from about 11 nm in absence of Pb to 6 nm with 2% of Pb-bearing, while 5% of Pb led to particles too small to be quantified with our TEM measurements. The combined effect of the decrease in particle size of magnetite, substitution of Pb and the precipitation of goethite in the Pb-dopped assay led to a significant decrease of magnetization at room temperature. Overall, this study highlights the effect of Pb on iron oxides bio-reduction and transformation processes and the sensitivity of magnetism to serve as a monitoring method.

Pb-Bearing Ferrihydrite Bioreduction and Secondary-Mineral Precipitation during Fe Redox Cycling

The significant accumulation of Pb from anthropogenic activities threatens environmental ecosystems. In the environment, iron oxides are one of the main carriers of Pb. Thus, the redox cycling of iron oxides, which is due to biotic and abiotic pathways, and which leads to their dissolution or transformation, controls the fate of Pb. However, a knowledge gap exists on the bioreduction in Pb-bearing ferrihydrites, secondary-mineral precipitation, and Pb partitioning during the bioreduction/oxidation/bioreduction cycle. In this study, Pb-bearing ferrihydrite (Fh_Pb) with various Pb/(Fe+Pb) molar ratios (i.e., 0, 2, and 5%) were incubated with the iron-reducing bacterium Shewanella oneidensis MR-1 for 7 days, oxidized for 7 days (atmospheric O2), and bioreduced a second time for 7 days. Pb doping led to a drop in the rate and the extent of the reduction. Lepidocrocite (23–56%) and goethite (44–77%) formed during the first reduction period. Magnetite (72–84%) formed during the second reduction. The extremely-low-dissolved and bioavailable Pb concentrations were measured during the redox cycles, which indicates that the Pb significantly sorbed onto the minerals that were formed. Overall, this study highlights the influence of Pb and redox cycling on the bioreduction of Pb-bearing iron oxides, as well as on the nature of the secondary minerals that are formed.

Single molecule tracking of bacterial cell surface cytochromes reveals dynamics that impact long-distance electron transport

Using a series of multiheme cytochromes, the metal-reducing bacterium Shewanella oneidensis MR-1 can perform extracellular electron transfer (EET) to respire redox-active surfaces, including minerals and electrodes outside the cell. While the role of multiheme cytochromes in transporting electrons across the cell wall is well established, these cytochromes were also recently found to facilitate long-distance (micrometer-scale) redox conduction along outer membranes and across multiple cells bridging electrodes. Recent studies proposed that long-distance conduction arises from the interplay of electron hopping and cytochrome diffusion, which allows collisions and electron exchange between cytochromes along membranes. However, the diffusive dynamics of the multiheme cytochromes have never been observed or quantified in vivo, making it difficult to assess their hypothesized contribution to the collision-exchange mechanism. Here, we use quantum dot labeling, total internal reflection fluorescence microscopy, and single-particle tracking to quantify the lateral diffusive dynamics of the outer membrane-associated decaheme cytochromes MtrC and OmcA, two key components of EET in S. oneidensis. We observe confined diffusion behavior for both quantum dot-labeled MtrC and OmcA along cell surfaces (diffusion coefficients DMtrC = 0.0192 ± 0.0018 µm2/s, DOmcA = 0.0125 ± 0.0024 µm2/s) and the membrane extensions thought to function as bacterial nanowires. We find that these dynamics can trace a path for electron transport via overlap of cytochrome trajectories, consistent with the long-distance conduction mechanism. The measured dynamics inform kinetic Monte Carlo simulations that combine direct electron hopping and redox molecule diffusion, revealing significant electron transport rates along cells and membrane nanowires.

Reversing Electron Transfer Chain for Light-Driven Hydrogen Production in Biotic-Abiotic Hybrid Systems.

The biotic-abiotic photosynthetic system integrating inorganic light absorbers with whole-cell biocatalysts innovates the way for sustainable solar-driven chemical transformation. Fundamentally, the electron transfer at the biotic-abiotic interface, which may induce biological response to photoexcited electron stimuli, plays an essential role in solar energy conversion. Herein, we selected an electro-active bacterium Shewanella oneidensis MR-1 as a model, which constitutes a hybrid photosynthetic system with a self-assembled CdS semiconductor, to demonstrate unique biotic-abiotic interfacial behavior. The photoexcited electrons from CdS nanoparticles can reverse the extracellular electron transfer (EET) chain within S. oneidensis MR-1, realizing the activation of a bacterial catalytic network with light illumination. As compared with bare S. oneidensis MR-1, a significant upregulation of hydrogen yield (711-fold), ATP, and reducing equivalent (NADH/NAD+) was achieved in the S. oneidensis MR-1-CdS under visible light. This work sheds light on the fundamental mechanism and provides design guidelines for biotic-abiotic photosynthetic systems.

Low-frequency seismic responses during microbial biofilm formation in sands

This study investigates changes in low-frequency attenuation responses of sands during microbial formation of soft viscous biofilms, or extracellular polymeric substances (EPS). The resonant column experiments were conducted with two model bacteria Shewanella oneidensis MR1 and Leuconostoc mesenteroides, while monitoring changes in the wave velocities and damping ratios associated with EPS formation in sands. The results show that the accumulation of soft, viscous EPS hardly changes the wave velocities, both the shear and flexural modes. By contrast, the low-frequency attenuations, both torsional and flexural damping ratios, show significant increases with the accumulation of highly viscous EPS. It is found that the contribution of EPS to seismic responses of water-saturated sands is mainly limited to the pore fluid component, causing additional energy dissipation during wave propagation, but with no or minimal impact on skeletal stiffness or no involvement in seismic stress transfer. With these unique and unprecedented low-frequency seismic data of biofilm-associated sands, the results suggest that the formation and accumulation of soft, viscous EPS or biofilms by bacterial activities can be detected by monitoring seismic attenuation and can also alter the seismic attenuation responses of sands, such as the case under earthquake loading or blast-induced compaction.

Relaxation behavior in low-frequency complex conductivity of sands caused by bacterial growth and biofilm formation by Shewanella oneidensis under a high-salinity condition

Complex electrical conductivity is increasingly used to monitor subsurface processes associated with microbial activities because microbial cells mostly have surface charges and thus electrical double layers. Although highly saline environments are frequently encountered in coastal and marine sediments, there are limited data available on the complex conductivity associated with microbial activities under a high-salinity condition. Therefore, we have developed the spectral responses of complex conductivity of sand associated with bacterial growth and biofilm formation under a highly saline condition of approximately 1% salinity and approximately 2 S/m pore water conductivity with an emphasis on relaxation behavior. A column test is performed, in which the model bacteria Shewanella oneidensis MR-1 are stimulated for cell growth and biofilm formation in a sand pack, whereas the complex conductivity is monitored from 0.01 Hz to 10 kHz. The test results indicate that the real conductivity increases in the early stage due to the microbial metabolites and the increased surface conduction with cell growth but soon begin to decrease because of the reduction of charge passages due to bioclogging. However, the imaginary conductivity significantly increases with time, and clear bell-shaped relaxation behaviors are observed with the peak frequency of 0.1–1 Hz, associated with the double-layer polarization of cells and electrically conductive pili and biofilms. The Cole-Cole relaxation model appears to capture such relaxation behaviors well, and the modeling results indicate gradual increases in normalized chargeability and decreases in relaxation time during bacterial growth and biofilm formation in the highly saline condition. Comparison with previous literature confirms that the high-salinity condition further increases the normalized chargeability, whereas it suppresses the phase shift and thus the imaginary conductivity. Our results suggest that the complex conductivity can effectively capture microbial biomass formation in sands under a highly saline condition.

Dielectric constants of organic pollutants determine their strength for enhancing microbial iron reduction.

Physicochemical properties are essential characteristics of organic compounds, which not only impact the fate of organic pollutants but also determine their application in biological processes. Here, we first found that the dielectric constants (ɛ) of organic pollutants negatively correlated to their strength for enhancing microbial Fe(III) reduction. Those with lower ɛ values than 2.61 potentially promoted the above process following the sequence carbon tetrachloride (CT) > benzene > toluene > tetrachloroethylene (PCE) due to their different ability to deprotonate the phosphorus-related groups on the outer cell membrane of iron-reducing bacteria Shewanella oneidensis MR-1 (MR-1). The stronger deprotonation of phosphorus-related groups induced more negative charge of cell surface and more strongly increased cell membrane permeability and consequently stimulated faster release of flavin mononucleotide (FMN) as an electron shuttle/cofactor for Fe(III) reduction. These findings are significant for understanding the biogeochemistry in multi-organic contaminated subsurface and providing knowledge for remediation strategies and current production.

Effects of Fe(II) source on the formation and reduction rate of biosynthetic mackinawite: Biosynthesis process and removal of Cr(VI)

• Coculture of IRB and SRB was used for biosynthesis of nanoFeS.. • Source of Fe(II) affected the formation of biosynthetic mackinawite. • Distribution of Fe(II) influenced the Cr(VI) removal capbility of mackinawite. • Biosynthetic mackinawite provided an excellent adsorbent for Cr(VI) removal. Biosynthetic mackinawite (FeS) has high removal efficiency for in situ remediation of heavy metal contamination. However, the effect of the iron (Fe) source on the efficiency of biosynthesized nano-FeS (nFeS) is less known. In this study, we examined the effect of Fe(II) produced i) directly from iron-reducing bacteria (IRB) Shewanella oneidensis MR-1 (HM group), and ii) indirectly from sulfate-reducing bacteria (SRB) Desulfosporosinus meridiei (DM group) on the biosynthesis of mackinawite, focusing on the biomineralization process and its capacity to remove hexavalent chromium (Cr(VI)). The HM group formed minerals around the cells, whereas the DM group formed minerals in the solution. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectrometry analyses further showed that the Fe(II) produced directly (by MR-1, HM group) and indirectly (reduced by biogenic H 2 S, DM group) led to a different distribution of Fe(II) in FeS minerals. The removal capacities of the produced HM–nFeS and DM–nFeS at pH 3 were the same, with a difference of approximately 20% with an increase in pH from 5 to 11, further indicating that the formed FeS products were different. The MR-1 (HM group) directly reduced Fe(III) and secreted a large amount of organic matter, which may have inhibited the ability of the surface to adsorb HCrO − and CrO 4 2− . These findings shed new light on the biosynthesis of mackinawite for bioremediation of heavy metals.

Electrical properties of outer membrane extensions from Shewanella oneidensis MR-1.

Shewanella oneidensis MR-1 is a metal-reducing bacterium that is able to exchange electrons with solid-phase minerals outside the cell. These bacterial cells can produce outer membrane extensions (OMEs) that are tens of nanometers wide and several microns long. The capability of these OMEs to transport electrons is currently under investigation. Tubular chemically fixed OMEs from S. oneidensis have shown good dc conducting properties when measured in an air environment. However, no direct demonstration of the conductivity of the more common bubble-like OMEs has been provided yet, due to the inherent difficulties in measuring it. In the present work, we measured the electrical properties of bubble-like OMEs in a dry air environment by Scanning Dielectric Microscopy (SDM) in force detection mode. We found that at the frequency of the measurements (∼2 kHz), OMEs show an insulating behavior, with an equivalent homogeneous dielectric constant εOME = 3.7 ± 0.7 and no dephasing between the applied ac voltage and the measured ac electric force. The dielectric constant measured for the OMEs is comparable to that obtained for insulating supramolecular protein structures (εprotein = 3-4), pointing towards a rich protein composition of the OMEs, probably coming from the periplasm. Based on the detection sensitivity of the measuring instrument, the upper limit for the ac longitudinal conductivity of bubble-like OMEs in a dry air environment has been set to σOME,ac < 10-5 S m-1, a value several orders of magnitude smaller than the dc conductivity measured in tubular chemically fixed OMEs. The lack of conductivity of bubble-like OMEs can be attributed to the relatively large separation between cytochromes in these larger OMEs and to the suppression of cytochrome mobility due to the dry environmental conditions.

Degradation of 2, 2′, 4, 4′-Tetrabrominated diphenyl ether (BDE-47) via the Fenton reaction driven by the dissimilatory metal-reducing bacterium Shewanella oneidensis MR-1

A microbially facilitated approach was developed to degrade 2, 2′, 4, 4′-tetrabrominated diphenyl ether (BDE-47). This approach consisted of biological production of Fe(II) and H2O2 by the dissimilatory metal-reducing bacterium Shewanella oneidensis MR-1 during the repetitive anoxic/oxic cycles and abiotic production of hydroxyl radical (HO●) with the biologically produced Fe(II) and H2O2 via Fenton reaction. Under the condition tested, BDE-47 did not inhibit the growth of S. oneidensis MR-1. Water soluble Fe(III)-citrate and the solid minerals ferrihydrite [Fe(III)2O3•0.5H2O] and goethite [Fe(III)OOH] were tested in this study. Under anoxic condition, the amounts of Fe(II) produced by S. oneidensis MR-1 varied among the Fe(III)s tested, which decreased in the order of Fe(III)-citrate > ferrihydrite > goethite. Under subsequent oxic condition, H2O2 was produced via O2 reduction by S. oneidensis MR-1. The amounts of H2O2 detected also varied, which decreased in the order of the reactions with Fe(III)-citrate > goethite > ferrihydrite. S. oneidensis MR-1 maintained its ability to produce Fe(II) and H2O2 for up to seven anoxic/oxic cycles. At each end of anoxic/oxic cycle, HO● was detected. The amount of HO● produced decreased in the order of the reactions with ferrihydrite > goethite > Fe(III)-citrate, which was opposite to that of H2O2 detected. Compared to the controls without HO●, the amounts of BDE-47 in the reactions with HO● decreased. The more HO● in the reaction, the less amount of BDE-47 detected. Furthermore, no BDE-47 degradation was observed when HO● was scavenged or ferrihydrite was either omitted or replaced by nitrate. Finally, identification of degradation products, such as hydroxylated BDE-47 and trisBDE, dibromophenol and monobromophenol, suggested that OH-addition and Br-substitution by HO● were the main mechanisms for degrading BDE-47. Collectively, all these results demonstrated for the first time that the Fenton reaction driven by S. oneidensis MR-1 degraded BDE-47 effectively.

Extracellular Electron Shuttling Mediated by Soluble c-Type Cytochromes Produced by Shewanella oneidensis MR-1.

How metal-reducing bacteria transfer electrons during dissimilatory energy generation under electron acceptor-limited conditions is poorly understood. Here, we incubated the iron and manganese-reducing bacterium Shewanella oneidensis MR-1 without electron acceptors. Removal of soluble extracellular organic compounds (EOCs) dramatically retarded transfer of electrons to an experimental electron acceptor, Cr(VI), by MR-1. However, the return of either high MW (>3000 Da) or low MW (<3000 Da) soluble EOCs produced by MR-1 to washed cells restored Cr(VI) reduction though Cr(VI) reduction was fastest when both size fractions were added together. Spectral and electrochemical characterization of EOCs indicated the presence of flavins and c-type cytochromes (c-Cyts). A model of the kinetics of individual elementary reactions between cells, flavins, released c-Cyts, and Cr(VI), including the direct reduction of flavins, released c-Cyts, and Cr(VI) by cells and the indirect reduction of Cr(VI) by reduced forms of flavins and released c-Cyts, was developed. Model results suggest that released c-Cyts could act as electron mediators to accelerate electron transfer from cells to Cr(VI), and the relative contribution of this pathway was higher than that mediated by flavins. Hence, extracellular c-Cyts produced by MR-1 likely play a role in extracellular electron transfer under electron acceptor-limited conditions. These findings provide new insights into extracellular electron shuttling and the metabolic strategy of metal-reducing bacteria under electron acceptor-limited conditions.